The context of the invention is that of a naval vessel or ship intended to tow a submersible object such as a variable-immersion sonar incorporated into a towed body. In such a context, in the non-operational phase, the submersible body is stored on board the ship and the cable is wound around the drum of a winch used for winding in and paying out the cable, namely for deploying and recovering the cable. Conversely, in the operational phase, the submersible body is submerged behind the ship and towed by the latter using the cable, of which the end connected to the submersible body is immersed. The cable is wound in/paid out by the winch through a cable guiding device that allows the cable to be guided.
In order to obtain a high degree of immersion at high towing speeds, the towing cable is faired to reduce its hydrodynamic drag and to reduce the vibrations caused by the hydrodynamic flow around the cable. The cable is covered with a segmented fairing made up of rigid fairing elements having shapes intended to reduce the hydrodynamic drag of the cable. The purpose of the sheath made up of the fairing elements is to reduce the wake turbulence produced by the movement of the cable through the water, when this cable is immersed in the water and towed by the ship. For great immersion depths that go hand-in-hand with high towing speeds of at least 20 knots, the fairing elements need to be rigid. Flexible fairings are of benefit only for economically profiling chains or cables for buoys subjected to marine currents or, at worst, towed at speeds of 6 to 8 knots. In the case of the use of rigid fairing elements, segmenting the fairing into fairing elements is necessary so that the cable can pass through guide elements of the pulley type, and so that lateral cable deflection can be tolerated in case the ship changes heading and also so as to be able to be wound onto the drum of a winch.
In the normal operating state, the fairing elements are mounted with the ability to rotate about the longitudinal axis of the cable. This is because it is necessary for the fairing elements to be able to rotate freely about the cable so as to be correctly oriented with respect to the stream of the water. However, each fairing element is connected to its two neighbors axially and in terms of rotation about the cable in such a way as to be able to pivot with respect to these about an axis parallel to the axis x by a maximum angle that is small, of the order of a few degrees. This link between the fairing elements in particular allows the fairing as a whole to pass fluidly through all the guide elements. As a result, the rotation of one of one fairing element leads to a rotation of its neighbors and so on and so forth through the entire set of fairing elements. Thus, both when the cable is deployed in the water and when it is wound around the drum, any change in orientation of one of the fairing elements has a knock-on effect on all of the fairing element fairing the cable. Thus, when the cable is deployed at sea, the fairing elements naturally orient themselves in the direction of the current generated by the movement of the vessel. Likewise, the guide device is conventionally configured to orientate and guide the fairing elements that pass through it in such a way that these exhibit a predefined orientation with respect to the drum of the winch, all the fairing elements, as the cable is raised, adopting one and the same orientation relative to the drum, which orientation allows the cable to be wound in keeping the scales parallel to one another from turn to turn.
Now, the applicant company has found that, when the faired cable is wound around the drum of a winch so as to recover the towed body, the fairing sometimes becomes severely damaged or even crushed as it passes through the guide devices, this being something which may render the entire sonar system unavailable. It may even happen that this damages the guide device. By way of example, certain variable-immersion sonar systems installed on certain ships and operated in the normal way by military crews encounter fairing-element-crushing problems approximately once a year and sometimes far more frequently. This crushing may have limited consequences but may also degenerate or jam the winch or damage it, and thus cause the entire towing system and therefore the sonar to become unavailable.